Precambrian tectonics lecture - Greenstone belts

General characteristics of greenstone belts:

Internal stratigraphy of greenstone belts:

Some examples and images of components:

Barberton belt in S. Africa is a classic and well studied example of a greenstone belt. Deformation and metamorphism or less intense here so the earlier basin history is clearer.

Eli greenstone belt of Minnesota: see below images and text.

This is an image of pillow basalts in the Eli greenstone belt in northern Minnesota. One can see the more pointed or tapered bottoms and the more bulbous tops of the pillows, indicating a younging to the left (South). The finer grained, once glassy rims are now a darker color, while the interior is weathered white. Glacial striations can also be seen on the outcrop surface. Camera lense in center for scale.

This is an image of a hypabyssal and phenocrystic dacitic intrusion in part of the Eli greenstone belt. To the left are the fine-grained metasediments the dacite intruded into. Note the small xenoliths in the dacite. Intermediate composition volcanics (dacites and andesites) are a common rock type in the area. Metamorphism is relatively mild and original igneous textures are preserved.

This is a glaciated flatrock exposure from the Eli greenstone belt literally just down the road from the above outcrop. Exposed are graded volcanic wackes layers intercalated with muds, suggesting these are turbidites in a basin next to a volcanic chain. They are upright here and elsewhere show evidence of both soft-sediment and tectonic deformation. One can also see a nice branching fault termination here.

Models for greenstone belts

Are they collapsed impact basins (doubtful)?

Are they accreted arcs?

Are they collapsed rear-arc basins?

Are they collapsed continental rifts?

Are they polygenetic?

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.